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ABSTRACT: We describe the growth of Zn(1-x)Mn(x)Se nanowires in ultrahigh vacuum seeded by Au nanodroplets. Electron microscopy reveals the formation of single-crystal c-axis wurtzite nanowires (typically 1-3 microm long) with Mn concentrations up to x approximately 0.6, accompanied by a dense horizontal undergrowth of shorter, crooked nanowires. Magnetophotoluminescence measurements show evidence for sp-d exchange effects in a reduced symmetry environment. We find that the optical emission is surprisingly dominated by the undergrowth of crooked nanowires.
Nano Letters 10/2009; 9(9):3142-6. · 13.20 Impact Factor
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ABSTRACT: Alloy disorder in II-VI diluted magnetic semiconductors (DMS) is typically reduced when the local magnetic spins align in an applied magnetic field. An important and untested expectation of current models of alloy disorder, however, is that alloy fluctuations in many DMS compounds should increase again in very large magnetic fields of order 100 tesla. Here we measure the disorder potential in a Zn$_{.70}$Cd$_{.22}$Mn$_{.08}$Se quantum well via the low temperature photoluminescence linewidth, using a new magnet system to 89 T. Above 70 T, the linewidth is observed to increase again, in accord with a simple model of alloy disorder. Comment: 4 pages, 3 figures
12/2006;
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ABSTRACT: We measure the low-field Hall resistivity of a magnetically doped two-dimensional electron gas as a function of temperature and electrically gated carrier density. Comparing these results with the carrier density extracted from Shubnikov-de Haas oscillations reveals an excess Hall resistivity that increases with decreasing temperature. This excess Hall resistivity qualitatively tracks the paramagnetic polarization of the sample, in analogy to the ferromagnetic anomalous Hall effect. The data are consistent with skew scattering of carriers by disorder near the crossover to localization.
Physical Review Letters 06/2006; 96(19):196404. · 7.37 Impact Factor
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ABSTRACT: We measure the low-field Hall resistivity of a magnetically-doped two-dimensional electron gas as a function of temperature and electrically-gated carrier density. Comparing these results with the carrier density extracted from Shubnikov-de Haas oscillations reveals an excess Hall resistivity that increases with decreasing temperature. This excess Hall resistivity qualitatively tracks the paramagnetic polarization of the sample, in analogy to the ferromagnetic anomalous Hall effect. The data are consistent with skew-scattering of carriers by disorder near the crossover to localization.
12/2005;
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ABSTRACT: Polarization-resolved magnetophotoluminescence spectroscopy is used to study exciton spin states in 40–80 Å diameter chemically synthesized CdSe quantum dots (QDs) at temperatures T=1.2–50K. The spin polarization is found not to saturate in magnetic fields to 60 T and time-resolved studies indicate a thermal population of exciton states. A simple model incorporating the angle-dependent Zeeman splitting and bright-dark level mixing in these randomly oriented quantum dots is constructed in quantitative agreement with the data. Fits using this model yield a dark exciton g factor of ∼0.9 at T=1.45K, which is independent of QD diameter and exhibits a surprising increase with increasing temperature.
Phys. Rev. B. 04/2001; 63(20).
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ABSTRACT: Low-temperature magneto-photoluminescence studies of negatively charged excitons (X- trions) are reported for n-type modulation-doped ZnSe/Zn(Cd,Mn)Se quantum wells over a wide range of Fermi energy and spin-splitting. The magnetic composition is chosen such that these magnetic two-dimensional electron gases (2DEGs) are highly spin-polarized even at low magnetic fields, throughout the entire range of electron densities studied (5e10 to 6.5e11 cm^-2). This spin polarization has a pronounced effect on the formation and energy of X-, with the striking result that the trion ionization energy (the energy separating X- from the neutral exciton) follows the temperature- and magnetic field-tunable Fermi energy. The large Zeeman energy destabilizes X- at the nu=1 quantum limit, beyond which a new PL peak appears and persists to 60 Tesla, suggesting the formation of spin-triplet charged excitons. Comment: 5 pages (RevTex), 4 embedded EPS figs. Submitted to PRB-RC
01/2000;
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ABSTRACT: Spin clustering in diluted magnetic semiconductors (DMS) arises from antiferromagnetic exchange between neighboring magnetic cations and is a strong function of reduced dimensionality. Epitaxially-grown single monolayers and abrupt interfaces of DMS are, however, never perfectly two-dimensional (2D) due to the unavoidable inter-monolayer mixing of atoms during growth. Thus the magnetization of DMS heterostructures, which is strongly modified by spin clustering, is intermediate between that of 2D and 3D spin distributions. We present an exact calculation of spin clustering applicable to arbitrary distributions of magnetic spins in the growth direction. The results reveal a surprising insensitivity of the magnetization to the form of the intermixing profile, and identify important limits on the maximum possible magnetization. High-field optical studies of heterostructures containing "quasi-2D" spin distributions are compared with calculation. Comment: 5 pages (RevTeX), 5 embedded EPS figs, published in PRB v61 p1736 (2000)
01/2000;
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ABSTRACT: We present experiments in the 60 T long-pulse magnet at the Los Alamos National High Magnetic Field Lab focusing on the high-field, low temperature photoluminescence (PL) from modulation-doped ZnSe/Zn(Cd,Mn)Se single quantum wells. High-speed charge-coupled array detectors and the long (2 s) duration of the magnet pulse permit continuous acquisition of optical spectra throughout a single magnet shot. High-field PL studies of the magnetic two-dimensional electron gases at temperatures down to 350 mK reveal clear intensity oscillations corresponding to integer quantum Hall filling factors, from which we determine the density of the electron gas. At very high magnetic fields, steps in the PL energy are observed which correspond to the partial unlocking of antiferromagnetically bound pairs of Mn2+ spins. © 1999 American Institute of Physics.
Journal of Applied Physics 04/1999; 85(8):5932-5934. · 2.17 Impact Factor
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ABSTRACT: We present experiments in the 60 Tesla Long-Pulse magnet at the Los Alamos National High Magnetic Field Lab (NHMFL) focusing on the high-field, low temperature photoluminescence (PL) from modulation-doped ZnSe/Zn(Cd,Mn)Se single quantum wells. High-speed charge-coupled array detectors and the long (2 second) duration of the magnet pulse permit continuous acquisition of optical spectra throughout a single magnet shot. High-field PL studies of the magnetic 2D electron gases at temperatures down to 350mK reveal clear intensity oscillations corresponding to integer quantum Hall filling factors, from which we determine the density of the electron gas. At very high magnetic fields, steps in the PL energy are observed which correspond to the partial unlocking of antiferromagnetically bound pairs of Mn<sup>2+</sup> spins.
11/1998
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ABSTRACT: Ultrafast optical pulses are used to initiate and measure free-induction decays of coherent conduction electron spins and of embedded magnetic Mn2+ ions in a series of magnetic-semiconductor quantum wells. These time-resolved Faraday rotation experiments in transverse applied magnetic fields complement previous studies of spin dynamics in longitudinal fields by unambiguously distinguishing between the spin relaxation of electrons and holes, and by identifying a mechanism by which angular momentum is transferred from spin-polarized carriers to the sublattice of local moments. In transverse fields (Voigt geometry), the precession of the photoexcited spins about the field axis can be measured as an oscillatory induced Faraday rotation signal. We observe the THz free-induction decay of spin-polarized electrons in modest (<4 T) magnetic fields and separately identify the more rapid spin relaxation of the holes as functions of field and temperature. The g factors of the electrons and holes are accurately measured as a function of well width. The role of quantum confinement on the stability of the hole spin is discussed, with particular attention given to the observed ability of the transient hole-exchange field to coherently rotate a macroscopic ensemble of local Mn2+ moments. This “tipping pulse” initiates a free-induction decay in the sublattice of Mn2+ spins and enables electron paramagnetic resonance (EPR) studies of the fractional monolayer magnetic planes. These time-domain EPR measurements reveal a significant magnetic field dependence of the Mn transverse spin relaxation time.
Phys. Rev. B. 09/1997; 56(12).
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ABSTRACT: We describe a method of reducing the timing jitter between two passively mode‐locked femtosecond titanium:sapphire laser systems, enabling femtosecond‐resolved measurements with independently tunable pump and probe wavelengths. The scheme supplements a commercially available system (Coherent Synchrolock) which locks two Ti:sapphire lasers to a master clock. By selecting only those pulses which are temporally coincident within a user‐specified range, a timing jitter reduction between the two lasers from ∼3 ps to ≪200 fs FWHM can be achieved, as measured by optical cross correlation. The timing jitter between the lasers is easily varied, allowing optimization of the tradeoff between temporal resolution and throughput depending on experimental needs. © 1996 American Institute of Physics.
Review of Scientific Instruments 07/1996; · 1.37 Impact Factor
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ABSTRACT: A time-resolved resonant Faraday rotation spectroscopy is employed
to study the dynamical interplay between local magnetic moments and
photoexcited carrier spins in quantum-confined semiconductor geometries.
This highly sensitive technique functions as an energy selective,
noninvasive, all-optical probe of spin dynamics ranging from femtosecond
to microsecond timescales and is particularly suited to low-dimensional
systems having small numbers of magnetic spins. Carrier spin-scattering
rates, lifetimes, and the orientation and relaxation of perturbed
magnetic ions are directly observed in the time domain. The utility of
this technique is demonstrated through the study of a newly developed
class of magnetic heterostructure, in which fractional monolayer planes
of magnetic Mn<sup>2+</sup> ions are incorporated
“digitally” into nonmagnetic II-VI ZnSe-ZnCdSe quantum
wells. These digital magnetic heterostructures (DMH) possess large
g-factors and exhibit enormous low-field resonant Faraday rotations in
excess of 1.7×10<sup>7</sup> deg/T·cm at low temperatures.
Time-resolved Faraday rotation measurements identify a wealth of
unexpected electronic and magnetic spin dynamics that are different from
those generated in traditional semiconductors or alloyed diluted
magnetic semiconductor structures
IEEE Journal of Selected Topics in Quantum Electronics 01/1996; · 3.78 Impact Factor
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ABSTRACT: Coherently strained CdSe quantum structures are fabricated under varying dynamical growth conditions during the epitaxy of cubic CdSe on (100) ZnSe. Reflection high energy electron diffraction (RHEED) is employed to monitor the growth mode (2D vs. 3D). Conventional photoluminescence (PL) shows that both growth modes yield quantum structures with high PL efficiencies in which excitons are strongly localized by interface fluctuations at varying length scales. Spatially-resolved, near-field PL from quantum structures formed during 3D growth reveals reproducible fine structure in the PL spectrum attributed to emission from excitons laterally confined to quantum dot-like regions. Transmission electron microscopy (TEM) studies suggest that these observations result from a combination of island growth and strain-driven interdiffusion.
MRS Proceedings. 12/1994; 417.
